The speed of metal-oxide-semiconductor (MOS) transistors is closely related to the drive currents of the MOS transistors, which drive currents are further closely related to the mobility of charges. For example, NMOS transistors have high drive currents when the electron mobility in their channel regions is high, while PMOS transistors have high drive currents when the hole mobility in their channel regions is high.
Compound semiconductor materials of group III and group V elements (referred to as III-V compound semiconductors hereinafter) are good candidates for forming transistors due to their high electron mobility. Therefore, III-V based transistors have been explored. III-V compound semiconductor films, however, need to be grown on other substrates because it is difficult to obtain bulk III-V crystals. The growth of III-V compound semiconductor films on dissimilar substrates faces difficulties because these substrates have lattice constants and thermal expansion coefficients different than that of the III-V compound semiconductors. Various methods have been used to form high quality III-V compound semiconductors. For example, III-V compound semiconductors were grown from trenches between shallow trench isolation regions to reduce the number of threading dislocations. Fin Field-Effect Transistors (FinFETs) are formed based on the resulting III-V compound semiconductors. Since PMOS and NMOS transistors have their channels and source and drain regions grown at the same time, the epitaxy processes of PMOS and NMOS transistors need to be separated, so that the well regions of the PMOS and NMOS transistors may have different conductivity types.